The present invention relates to a method for digital data transmission with a variable bandwidth in a network which has at least one server, a user terminal and a so-called network management system (TMN), the method enabling flexible service-dependent bandwidth control.
The invention uses the so-called ADSL (Asymmetric Digital Subscriber Line) system, which has recently become more and more widespread. The ADSL system is described for example in the documents by Veeneman, Olshansky, GTE Laboratories Incorporated, “ADSL for Video and Data Services”, IEEE Communications Conference, 1995, pages 837-841; or Chow, Cioffi, Amati Communications Corporation, “A Multi-drop ADSL Distribution Network”, IEEE Communications Conference, 1994, pages 456-460.
In this asymmetric transmission system, the data stream from a user terminal to a server is significantly smaller than the opposite data stream, that is to say the data stream from the server to the user terminal. Typically, the data stream from the user terminal to the server amounts to a few kilobits per second, whereas the data stream from the server to the user terminals may amount to one or more megabits per second (for example 6 megabits/s in the case of standard telephone lines). The upper limit is prescribed in this case by the line conditions. ADSL is therefore particularly suitable for so-called video on demand or WWW applications, in which generally the data to be transmitted from the user terminal to a server are significantly fewer than in the opposite direction.
ADSL uses a modulation scheme in order to transmit data in particular to user terminals of private customers by means of the same twisted copper lines which are also used for ISDN services, for example. As a result, the installation costs for this new system are comparatively low for the network operator. It can therefore be used together with conventional telephone services, but, by contrast, it has the advantages of significantly higher transmission rates, namely about 6 to 8 megabits per second from the server to the user terminal and up to 640 kilobits per second from the user terminal to the server.
An essential property of the ADSL system in comparison with the ISDN system, for example, is that the modems at the user and operator ends have to be physically connected to one another (for example by means of a widely distributed copper line) and cannot simply be present respectively at one end of an interconnected and switched telephone connection. Consequently, one modem must usually be situated at a central distributor of the telephone company operating the ADSL system, and the other modem is correspondingly situated in the user's building. The modem which is connected in the switching station (central distributor) of the telephone company is called ATU-C (ADSL Terminal Unit Central). The modem which is situated at the user's end is called ATU-R (ADSL Terminal Unit Remote).
A further property of the ADSL system is that the bandwidth can be set by the operator. At present, in ADSL systems the bandwidth is set at the operator end in a manner governed by the system, i.e. the bandwidth is determined in accordance with the capabilities of the subscriber line used for transmission. In this method, which is also called rate-adaptive ADSL, the maximum possible bandwidth is set independently by the system. Thus, bandwidth is always used, which is generally prescribed by the line conditions.
EP 0 806 852 A2 discloses a multimode digital modem for ADSL transmissions. In an initialization method, the two modems involved at both ends of a line carry out a “negotiation method” (Rate Negotiation Method). Each of the modems involved communicates to the respective other modem its possibilities and desires with regard to the data transmission rate to be chosen for a transmission. The data transmission rate actually used during the transmission is then fixed according to an algorithm.
WO96/37069 discloses a video conference and multimedia system. A digital data transmission can be effected with different bandwidths, the currently used bandwidth being displayed.